Lincoln gave the slaves the right of self-defense. It SEEMS this was to back up a major change in the war that according to historian Richard Slotkin, was originally to preserve the union, but now–perhaps largely due to the “narcissistic” and insurrectionist demands of Lincoln’s pro-slavery general, West Point engineer, George McClellan, was to change the nature of the South and put men against the pro-slavery South.
McClellan had threatened the dissolving of the army and demanded in a letter that nothing radical be done to slavery.
Lincoln declared the emancipation in defiance of his intrigue-making military leader, who had thought of leading a dictatorship.
Declaring the freedom of the slaves is significant, because at the beginning of the war, the decisive end of slavery was not actually a key goal of the North, though Lincoln was morally opposed to it. In fact, in his efforts to lessen the conflict and keep the union together, Lincoln promised the South that it could keep slaves (though he envisioned ending slavery later). Check NPR for the interview I learned this in by looking up the author and his new book, ‘The Long Road to Antietem’.